The present invention relates, in general, to coating methods and, in particular, to a new and useful method for coating a surface with corrosion resistant nickel oxide. Tubes, pipe, or plate provided with the nickel oxide coating produced according to the present invention are particularly suited for service environments in refuse boilers burning processed and bulk municipal waste. The fuels burned in such refuse boilers are substantially higher in chlorine content, thus producing corrosive conditions much different than that typically encountered in utility power boilers burning coal and/or oil.
Corrosion resistant surface coatings on substrates can be produced by many different methods. For instance, coatings may be applied by painting, thermal spray, or metallurgical bonding methods. The method used generally depends on several factors such as the coating material being applied, the substrate material, the required coating integrity, and the required coating/substrate bond strength. The applied coating material in the known methods is generally put into service in the as-applied state.
Fine NiO powders have been produced for the ceramic industries through one known technique using the pyrolysis of an aerosol of a nickel nitrate solution.
This method is described in Gadalla et al., "Thermal Behavior of Ni(II) Nitrate Hydrate and Its Aerosols", Journal of Thermal Analysis, Vol. 37 (1991) 319-331 and Messing et al., "Synthesis of Ceramic Powders From Metal Alkoxides", Journal of the Ceramic Society of Japan, The Centennial Memorial Issue 99 [10] 1036-1046 (1991).
Duggan (U.S. Pat. No. 4,658,761) discloses a prior art technique used in the treatment of boiler tubes to improve their corrosion and erosion resistance. Metallized or non-metallized coatings are impregnated with at least one stable metal oxide by solutions containing salts or oxides of such metals. Preferably, a metal boiler tube having at least a selected part of its surface initially porous is coated with at least one stable metal oxide by the application to the porous surface of a solution or suspension containing salts or oxides of such metals followed by conversion of such salts or treatment of such oxides of metals to attach the stable metal oxides to the porous surface. A slurry may be used which is a liquid based mixture of one or more finely divided refractory oxides which may optionally contain a small amount of impregnating solution, a small amount of organic wetting agent, or ceramic reinforcement fibers. The slurry itself may contain a high percentage of metal powder including chromium or nickel-chromium alloy, and other finely powdered materials of high abrasion or corrosion-resisting capacity, for example silicon carbide, boron carbide, and titania-lead glass. The slurries may be applied directly to the metal surface. Additionally, preferred metal coatings to be applied according to the Duggan patent may be selected from the following: nickel-chrome alloy, nickel-aluminide alloy, and high chrome iron alloy. The metal coating may serve the role of a stress-relieving layer upon which a further layer, comprising oxide or metal, may be applied. Suitable compounds capable of conversion to stable metal oxides and which are soluble include, for example, cerrous nitrate, zirconyl chloride, cobalt and nickel nitrates, titanium oxalate, silico-tungstic acid, magnesium chromate, beryllium nitrate, chromium trioxide, chromium sulphate, chromium chloride, and the like. Finally, the application to the porous tube surface of a concentrated solution of chromic acid is particularly preferred, and results in a chromium oxide protective layer which is an effective corrosion inhibitor when sulfur compounds are the predominant corrosion agent, for example coal ash corrosion in utility boilers. The chromium also helps to resist normal oxidation. However, chromium compounds are not resistant to chloride-bearing corrodents such as those which occur in refuse boilers.
The corrosive environment typically encountered in coal and/or oil fired boilers of electric utility generating stations is significantly different from that encountered in refuse fired boilers burning processed and bulk municipal waste. The reason for this difference is the type of fuel being burned and the corrosive agents within that fuel. Chlorine exists in some coals in sufficient quantities to contribute, with the sulfur in the coal, to increased corrosion attack of boiler materials. However, the amount of chlorine in refuse is significantly higher, and the sulfur is generally usually very low. This creates a corrosion condition that is much different than that encountered in power boilers. In fact, some of the coatings that help protect against corrosion in power boiler environments are usually based upon increased chromium content materials or coatings, and thus do not work in the refuse boiler environment. An additional corrodent found in refuse that does not appear to any significant degree in coal or oil are low melting point elements, such as lead an zinc. These elements form low melting point eutectic compositions when combined with chlorine, and which are very corrosive to normal iron-based boiler materials. The most effective material found to consistently resist attack in refuse boiler environments are nickel and nickel-oxide based systems. Chromium oxides, the preferred embodiments of the Duggan patent, will simply not provide any significant degree of corrosion protection in a refuse fired boiler burning processed and bulk municipal waste.